1. Field of the Invention
The invention relates to a method for producing polyamide membranes with improved hydrolytic stability and to polyamide membranes with improved hydrolytic stability.
2. Description of Related Art
Polyamide membranes are used in membrane technology in many ways, e.g., as ultrafiltration membranes or microfiltration membranes for filtration purposes in such sectors as the food and drink industry or the electronics industry.
A wide variety of methods is available for production of polyamide membranes. The majority of these use wet-spinning processes, i.e., methods in which a solution of a polyamide in a solvent, to which a proportion of non-solvent can also be added, is brought into contact with a non-solvent. The membrane structure is then formed by the ensuing coagulation induced by the non-solvent.
A process of this type is described in, e.g., U.S. Pat. No. 4,340,479, DE-A-30 28 213 and DE-A-31 38 525, where formic acid is used as the solvent and water as the non-solvent or coagulant. U.S. Pat. No. 3,876,738 is based on a similar process and mentions the possibility of interposing a short air evaporation zone between the extrusion of the polymer solution and the coagulation bath, i.e., of using a dry-wet spinning process. A process of this type is described also in DE-A-25 54 922. EP-A-0 413 552 discloses a wet spinning process by means of which asymmetric polyamide membranes with cavern-shaped pores are produced. An alcohol/salt solution is used as the solvent.
Another method for producing microporous polyamide membranes is based on a process involving thermally induced phase separation. In this method, a homogeneous melt solution of the polymer in a solvent system is first prepared at elevated temperatures, wherein the polymer component and the solvent system form a binary system that in the liquid state (of aggregation) has a range in which it exists as a homogeneous solution, and a range in which it possesses a miscibility gap. When such a system is cooled below the demixing temperature, phase separation occurs and a porous polymeric structure is finally formed. Methods of this type in which polyamides, among other compounds, can be used as membrane-forming polymers are described in, e.g., DE-A-32 05 289, EP-A-0 133 882 and EP-A-0 309 136.
However, polyamide membranes produced by these known methods have generally limited suitability for use in applications where they come into contact with aqueous media, particularly at high temperatures and in the presence of oxygen. Degradation of the polyamides occurs in applications of this type. The polyamide membranes therefore have only limited service life in these applications; after some time, as a result of polymer degradation, the mechanical stability of the membrane can no longer be guaranteed and the membranes disintegrate. It must also be noted that the superheated steam sterilisation that is often necessary for the above applications leads to loss of the mechanical stability of the polyamide membranes after only a few sterilisation cycles, rendering the membranes unfit for use.
The need therefore exists for a method of producing polyamide membranes with improved hydrolytic stability, and for such polyamide membranes with improved hydrolytic stability, having a longer service life and higher stability to superheated steam sterilisation as compared with known polyamide membranes and those produced by known methods, for the above-mentioned applications where the membrane is in contact with aqueous media at high temperatures and in the presence of oxygen.